Method for producing directly reduced, desulphurized iron

ABSTRACT

The invention relates to a method for producing directly reduced, desulfurized iron in a multiple-hearth furnace which comprises two zones arranged one above the other and each having several hearths. Iron ore is reacted with a reducing agent in a first zone of the multiple-hearth furnace at a temperature ranging from 800° C. to 1100° C. to obtain metallic iron. In addition, the gases are desulfurized using desulphurizing agents, whereby the directly reduced iron is discharged from the multiple-hearth furnace and the desulfurized gases are guided into a second zone where the iron ore is preheated to a temperature ranging from 600° C. to 800° C.

The invention relates to a process for production of directly reduceddesulphurised iron.

The production of directly reduced iron takes place in a directreduction process by the reduction of iron oxide with solid or gaseousreducing agents. Coal, for example, which reacts with carbon dioxide athigher temperatures and forms the reducing gas CO, serves as a solidreducing agent. As the coal contains a relatively large amount ofsulphur and the iron ore used often also contains this element, both theiron produced and the waste gases have an excessively high sulphurcontent. Subsequent desulphurisation of the waste gases and the directlyreduced iron is expensive.

Task of the Invention

The task of the invention is to propose a process for production ofdirectly reduced, desulphurised iron.

General Description of the Invention

According to the invention this problem is solved by a process forproduction of directly reduced, desulphurised iron in a multiple-hearthfurnace, which has two zones one above the other, each with severalhearths, iron ore reacting with a carbon carrier at a temperaturebetween 800° C. and 1100° C. and becoming metallic iron and the gasesbeing desulphurised with desulphurising agents in a first zone of themultiple-hearth furnace, the directly reduced iron being discharged fromthe multiple-hearth furnace and the desulphurised gases conducted intothe second zone, where they preheat the iron ore to between 600° C. and800° C.

The sulphur content of the gases is reduced within the multiple-hearthfurnace by this process. With direct reduction of iron ore sulphur isreleased during gasification of the reducing agent, e.g. coal. A smallerquantity of sulphur may also be released during the reduction from ironore to iron. This sulphur is bound in this process within the first zoneand can therefore no longer react, or react only to a lesser extent,with the iron ore and the iron formed. Hence the directly reduced ironproduced according to this process has a clearly lower sulphur content.

The desulphurising agents contain, for example, lime (CaO), limestone(CaCO₃) and/or magnesite (MgO). The desulphurisation of the gasesadvantageously takes place largely within this first zone by chemicalreaction of the gases containing sulphur with these desulphurisingagents, the sulphur reacting with the calcium and/or magnesium to formsulphates, sulphites, sulphides and the like. These compounds are formedpreferably on the surface of the desulphurising agent. An advantage ofthese agents is that they have a positive effect on the slag propertieswhen they are melted together with the directly reduced iron.

The multiple-hearth furnace has a first zone and a second zone, whicheach comprise several hearths. The second zone advantageously lies abovethe first zone and the gases rise from the first zone to the second one,whereas the solids are gradually transferred from the second zone intothe first zone.

The solids, i.e. the iron ore and reducing agent, can be introducedseparately or together into the multiple-hearth furnace. According to apreferred embodiment the iron ore is first charged to the top hearth ofthe multiple-hearth furnace in the second zone. It is circulated thereby rakes, which project over the hearths, and gradually transferred tothe underlying hearths. The reducing agent is then charged to a hearthfurther down in the multiple-hearth furnace, preferably in the lowersection of the second zone, and mixed with the preheated iron ore.Agglomeration of the reducing agent and the ore is prevented by thecontinuous circulation. The reducing agents are, for example, bituminouscoal, lignite, coke, etc.

According to a preferred embodiment desulphurising agents and a reducingagent are mixed with the iron ore in the second zone.

The desulphurising agents can be introduced together with the iron oreand/or the carbon carrier into the multiple-hearth furnace. However,they can also be introduced separately into the multiple-hearth furnace.

Depending on the sulphur content of the reducing agent thedesulphurising agents can either be fed at one point into themultiple-hearth furnace or distributed and charged at various points. Inthe latter alternative, different particle sizes can also be used. Ithas proved to be advantageous to charge coarse-grained desulphurisingagents in the second zone and powdered desulphurising agents in thefirst zone. Depending on the granulometry of the particles the latterare discharged with the gases or solids from the multiple-hearthfurnace. Powdered desulphurising agent is advantageously introduceddirectly into the gas flow in the first zone, with the result that thedesulphurisation is more effective. The powdered desulphurising agentsenriched with sulphur are largely discharged with the gases from themultiple-hearth furnace.

The solids, i.e. the iron formed, the residues of the carbon carrier andthe desulphurising agents are discharged from the multiple-hearthfurnace and subsequently melted. The desulphurising agents together withthe gangue of the ore form a slag, and the combined sulphur is disposedof together with the slag.

The reduction gases in the furnace can be adjusted to an optimumconcentration by selective feeding of reducing agents into the lowerhearths of the multiple-hearth furnace and thus a better degree ofmetallisation achieved.

In addition a gaseous reducing agent can be injected into the bottomhearths of the multiple-hearth furnace. Consequently more completereduction of the ore is achieved.

All rising gases, including the volatile constituents of the reducingagents, can be subsequently burnt in the upper part of the furnace, i.e.in the second zone, and the residual heat of the gases inside thefurnace can thus be utilised in an optimum manner. Good thermalefficiency is accordingly achieved, because the energy yield is better.

The multiple-hearth furnace can be operated under a specificoverpressure to achieve a further increase in productivity.

According to a preferred embodiment the ore in the topmost hearths ofthe furnace is dried and preheated by the hot gases in the furnace andin contact with the hearths before it comes into contact with thereducing agent. The ore is heated to a temperature of at least 400° C.,preferably to at least 600° C. to 700° C., before the solid reducingagent is added.

Gases containing oxygen can be injected selectively on the hearth, wherethe heat requirement must be covered by combustion of the excess processgases, e.g. above the hearth, where the solid reducing agents areintroduced.

It is advantageous to use gases containing oxygen which have atemperature of at least 350° C.

In addition one or more hearths in the furnace which are below thehearth to which the solid reducing agents are introduced can be heatedby burners.

DESCRIPTION WITH THE AID OF THE FIGURES

An embodiment of the invention will now be described below with the aidof the enclosed figures.

FIG. 1 is a section through a multiple-hearth furnace for production ofdirectly reduced, desulphurised iron.

FIG. 1 shows a section through a multiple-hearth furnace 10, which hasseveral hearths 12 one above the other. These unsupported hearths 12, aswell as the shell 14, cover 16 and bottom 18 of the furnace are madefrom refractory material.

The multiple-hearth furnace 10 is divided into two zones one above theother, viz. a first zone 20 and a second zone 22.

An exhaust 26, through which the gases can be evacuated from thefurnace, and an opening 28, through which the ore can be charged to thetop hearth, are provided in the cover 16 of the furnace 10.

A shaft 24, on which rakes projecting over the respective hearths arefitted, is installed in the centre of the furnace 10.

The rakes are designed in such a way that they circulate the material onone hearth from the inside outwards and then on the underlying hearthfrom the outside inwards in order to transport the material in this wayfrom the top downwards through the furnace. The shaft 24 and the rakesare air-cooled, and openings, through which the air can flow into thefurnace interior and be used there for after-combustion, are provided onthe rakes.

After the ore is charged to the first hearth, it is circulated by therakes and conveyed to the edge of the hearth, from where it fallsthrough several openings provided for this purpose on to the underlyinghearth. The ore is conveyed to the centre of the hearth and then fallson to the underlying hearth. During this time the ore is dried bycontact with the hearth and the rising hot gases and heated to about600° C.

Inlet openings 30, 32, 34, through which solid materials are introducedinto the furnace, are provided in the side walls of the multiple-hearthfurnace 10. These solid materials are firstly carbon carriers such aslignite coke, petroleum coke, blast furnace dust, coal or the like and,secondly, desulphurising agents such as lime (CaO), limestone (CaCO₃)and/or magnesite (MgO).

The carbon carrier is introduced to a hearth through openings 30, 34into both zones 20, 22 and mixed with the heated ore by the rakes 22.The coal is gasified by the high temperature and carbon monoxide isformed, with the result that the iron oxide present in the ore isgradually reduced to metallic iron during transport through themultiple-hearth furnace 10.

The desulphurising agents are introduced into the multiple-hearthfurnace at several points. Coarse-grained desulphurising agent isintroduced together with the iron ore through the opening 28 and/ortogether with carbon carriers through the openings 30 into themultiple-hearth furnace.

In addition powdered desulphurising agent is injected into the risinggases through the opening 32 in the first zone 20.

In this process sulphur is released in two stages. Up to approximately600° C. the “mineral” sulphur, which is present in the ore or carboncarrier in the form of CaSO₄ and FeS₂ is released first in the form ofH₂S. Above a temperature of about 800° C. the sulphur present in thecarbon carrier in “organic” form is then released. Under the conditionsprevailing in the multiple-hearth furnace 10 this sulphur reactspreferentially with the desulphurising agents and can thus not reactwith the iron ore and iron. The gases are accordingly desulphurised andthe iron produced contains less sulphur than in conventional processes.

Nozzles 38 for injection of hot (350° C. to 500° C.) gases containingoxygen, through which air or another gas containing oxygen can be fedinto the furnace 10, are provided in the side wall. As a result of thehigh temperatures and the presence of oxygen some of the carbon is burntto carbon dioxide, which in turn reacts with the carbon present inexcess and is converted to carbon monoxide. The carbon monoxide finallyreduces the iron oxide to metallic iron. As this reaction ispredominantly endothermal, it is logical to install in the lower part ofthe furnace burners 40, which ensure a uniformly high temperature in thebottom hearths of the furnace. Gas or pulverised coal burners can beused here.

These burners can be fired with gas or pulverised coal with air forpreheating and/or additional heating. An additional reduction gas can beproduced by the quantitative ratio between oxygen and fuel, or in theevent of excess air after-combustion of the process gases is achieved.In the case of pulverised coal firing an excess of carbon monoxide canbe produced in the burner. With external combustion chambers the ashfrom the burnt coal can be prevented from entering the furnace andmixing with the directly reduced iron. The consumption of carboncarriers in the furnace 10 and thus also the ash content in the endproduct are reduced by the production of carbon monoxide.

In the final or final two stages provision is made for feeding a gaseousreducing agent, e.g. carbon monoxide or hydrogen, through specialnozzles 42. In this oxygen-free atmosphere the reduction of the ore canbe completed.

The directly reduced iron is subsequently discharged together with theash from the reducing agents through the outlet 44 in the bottom 18 offurnace 10.

The controlled feed of solid and gaseous reducing agents, gasescontaining oxygen and desulphurising agents at various points of themultiple-hearth furnace 10 permits accurate control of ore reduction andrealisation of the process under optimum conditions.

What is claimed is:
 1. A process for producing directly reduced iron,the process comprising providing a multiple-hearth furnace comprising afirst zone below a second zone, where each of the first zone and thesecond zone includes several hearths; feeding an iron ore into thesecond zone; feeding a reducing agent into at least one of the firstzone and the second zone; forming a gas comprising sulfur from sulfur inat least one of the iron ore and the reducing agent; contacting the gascomprising sulfur with a desulphurising agent to form a desulphurisedgas; preheating the iron ore in the second zone with the desulphurisedgas to a temperature between 600° C. and 800° C. to form a preheatediron ore; transferring the preheated iron ore from the second zone tothe first zone; and reacting the preheated iron ore in the first zone ata temperature between 800° C. and 1100° C. to form the directly reducediron.
 2. The process according to claim 1, wherein the desulphurisingagent contains at least one of lime, limestone, and magnesite.
 3. Theprocess according to claim 1, wherein the reducing agent is a solidreducing agent; and the process further comprises drying the iron ore inthe second zone and subsequently heating the dried iron ore to atemperature of at least 600° C. before adding the solid reducing agentto the dried iron ore in the second zone.
 4. The process according toclaim 1, further comprising feeding the desulphurising agent into thesecond zone.
 5. The process according to claim 1, further comprisingfeeding a first portion of the desulphurising agent into the first zone;and feeding a second portion of the desulphurising agent into the secondzone.
 6. The process according to claim 5, wherein the first portion ofthe desulphurising agent has a different granularity than the secondportion of the desulphurising agent.
 7. The process according to claim5, wherein the first portion of the desulphurising agent is powdered,and the second portion of the desulphurising agent is coarse-grained. 8.The process according to claim 1, wherein the reducing agent is a solidreducing agent; and the process further comprises drying the iron ore inthe second zone and subsequently heating the dried iron ore to atemperature of at least 400° C. before adding the solid reducing agentto the dried iron ore in the second zone.
 9. The process according toclaim 1, further comprising injecting gases containing oxygen into thefirst zone.
 10. The process according to claim 1, wherein the first zoneincludes a first bottom hearth at the bottom of the first zone; thesecond zone includes a second bottom hearth at the bottom of the secondzone; the reducing agent is at least one of a solid reducing agent and agaseous reducing agent; and the process further comprises introducingthe least one of a solid reducing agent and a gaseous reducing agentinto at least one of the first bottom hearth and the second bottomhearth.
 11. The process according to claim 1, further comprising heatingone or more hearths in the multiple-hearth furnace with burners.
 12. Theprocess according to claim 1, wherein the process takes place underoverpressure.